Blood clots like the one that Secretary of State Hillary Rodham Clinton is being treated for following her recent concussion can occur for a host of reasons.

How serious a clot is depends on where it is and why it formed. In a statement Monday, Clinton's doctors say her clot was located in a vein in the space between the brain and the skull behind the right ear

WHAT THEY ARE: Blood pools and thickens into a clot after an injury or because of a heart problem, clogged arteries or other condition. Clots also can break off and travel to another part of the body.

WHERE THEY OCCUR: In leg veins (called deep vein thrombosis) or in blood vessels in the neck, brain or lungs. Leg clots are a common risk after someone has been bedridden. Clots are most dangerous when they travel to the lungs, a potentially life-threatening situation, or to the brain, where they can cause a stroke.

RISK FACTORS: High blood pressure, diabetes, birth control pills, pregnancy, stroke, recent surgery, prolonged sitting, circulation problems and heart problems — especially an irregular heartbeat called atrial fibrillation — raise the chances of developing a blood clot.

TREATMENT: Sometimes a blood thinner such as warfarin (Coumadin) is prescribed to allow the clot to dissolve by itself over time and prevent new ones from forming.